The present invention relates to a radio communication system adopting a time-division multiple access (TDMA) method, and especially to a receiver having functions for canceling a DC offset of an input signal of a digital radio receiver and measuring a carrier detection threshold value, and a control method thereof.
Conventionally, in this kind of DC offset canceling method, in order to suppress deterioration of reception characteristic of a received signal due to a DC offset generated inside of an analog circuit by a noise, a temperature drift and so forth, in JP-A-242262/1996 for example, by providing a DC correcting circuit in the analog circuit, and further, by providing a switch for preventing an unnecessary received wave and a noise, a DC component is canceled before the received signal comes.
Also, in other example, as disclosed in JP-A-13482/1998, a DC offset removing circuit is arranged in an analog circuit and a digital circuit.
In addition to the above-described example, as an example in which a DC offset is canceled during reception of a burst, as disclosed in JP-B-36504/1994 for example, an arrangement in which a DC component is canceled by a capacitor of an analog circuit is proposed.
On the other hand, as a carrier detection method, it is known that a received signal strength indicator (RSSI) obtained by applying A/D conversion to an output of a log-amplifier in an analog circuit is compared with a preset threshold value, and in case that the RSSI exceeds the threshold value, a carrier detecting signal is output and used as a trigger of a received signal.
However, if the preset threshold value is set lower, due to an interference wave from other system, a received noise and so forth, a carrier detection signal is erroneously detected, and accordingly, there is a task that power is wasted and a received signal to be originally received is missed.
Also, to the contrary, if the preset threshold value is set higher, a received signal having the low RSSI cannot be received, and accordingly, a receiving area is narrowed.
The first task is that in the method of detecting a DC offset before a received signal, in which the DC offset generated in an analog circuit is detected by preventing an unnecessary received wave and a noise, the unnecessary received wave and noise cannot be sufficiently prevented by a switch of an RF analog circuit.
The reason thereof is that the switch cannot sufficiently block the unnecessary received wave and noise, and a correct DC offset cannot be detected due to a round path.
The second task is that, in the method in which the DC offset removing function is arranged in the analog circuit, the analog circuit becomes to be a complicated one, and also, increase of wasted power is introduced.
The reason thereof is that, in the DC offset circuit based on analog processing, the number of parts becomes to be large.
The third task is that, in the method in which the DC offset removing function is arranged in both the analog circuit and the digital circuit, it is not possible to constitute a small-sized and light receiver.
The reason thereof is that, since the number of parts becomes to be large, the circuits become to be complicated ones. The fourth task is that, in the method in which the DC offset is canceled in the digital circuit during a received burst, transmission efficiency becomes to be low.
The reason thereof is that, since extra training bits for canceling the DC offset have to be added to a head of a burst signal, an overhead other than an original data is increased.
The fifth task is that, in the carrier detection using a preset threshold value, the carriers are erroneously detected and a receiving area is narrowed. Also, it is difficult to set the threshold value in accordance with use environment. The reason thereof is that, since the threshold value is fixed, the carriers are erroneously detected due to affection by an external noise and unnecessary radio waves, and in order to take a margin for an erroneous detection of the carriers, the threshold value is set higher and the receiving area is unnecessarily narrowed.
The sixth task is that, in the case of the carrier detection method in which a threshold value is adaptively changed in accordance with reception environment, when the threshold value is measured, a power supply of the mixer included in the analog circuit cannot be sometimes cut when the DC offset detection is conducted.
The reason thereof is that, in case of taking out an RSSI from a log-amplifier taken out from an output of the above-described mixer, since the power supply of the mixer is turned off, a noise level of external environment cannot be obtained from the RSSI, and accordingly, the normal threshold value cannot be measured.
The objective of the present invention is to provide a receiver and a control method capable of detecting a high accurate DC offset.
The another objective of the present invention is to provide a cancellation circuit of a DC offset, which has a small circuit scale.
The other objective of the present invention is to provide a high efficient carrier detection system in which an erroneous detection is less and a receiving area can be broaden.
The other objective of the present invention is to provide a digital radio receiver with high quality and high reliability, in which the high accurate cancellation system of the DC offset, which has a small circuit scale, and the high efficient carrier detection system are combined.
In order to accomplish the above-described objectives, a receiver of the present invention adopts an arrangement having control means for generating a DC offset gate signal, a mixer power supply isolating signal and an RSSI gate signal based on a frame synchronizing signal, switch means for switching on and off a power supply of a mixer by means of control of the above-described mixer power supply isolating signal, DC offset cancel means for canceling a DC offset by means of control of the above-described DC offset gate signal, and carrier detecting means for measuring a threshold value for carrier detection by means of control of the above-described RSSI gate signal, and for determining whether a carrier exists.
A control method of the present invention adopts an arrangement in which, in a receiver having DC offset cancel means and carrier detection threshold value measuring means, the mixer power supply isolating signal and the DC offset gate signal output from the above-described control means are output at a TDMA frame unit alternately with the RSSI gate signal.
In the present invention, since the power supply of the mixer is turned off within a guard time period between transmitted and received bursts when the DC offset is detected, it is possible to cut unnecessary radio waves from an antenna input and a noise occurred in an analog circuit, and to detect the originally required DC offset.
Also, since the DC offset is detected within a guard time period between the transmitted and received bursts, it is not necessary to add extra training bits before the bursts only for the DC offset detection.
Moreover, since a threshold value for the carrier detection is measured within a guard time period between the transmitted and received bursts and is adaptively changed, compared with a fixed threshold value, a distance at which the carrier can be substantially detected is extended, and as a result, it is possible to broaden a communication area, and also, to prevent an erroneous detection of the carrier due to a noise and interference with other system.
Furthermore, it is possible to realize a receiver in which, by alternately conducting the cancellation of a DC offset and the measurement of a carrier detection threshold value at a TDMA frame unit, both the functions are built in.